Important unanswered questions surround the nature of body composition and resting energy expenditure (REE) after substantial and sustained weight loss by severely obese persons. The LABS population is well suited for the study of these questions in that: they are of a degree of fatness rarely studied, which allows us to test and extend the range of applicability of current knowledge; they will undergo massive weight loss; the weight loss is likely to be sustained over a long period of time thereby allowing for adaptations of organ and tissue mass and fat redistribution to occur; the surgical interventions achieve their effects by different degrees of restriction and malabsorption, providing an opportunity to determine whether and how this dimension affects body composition and thermogenesis. The composition of weight loss under usual negative energy balance conditions in overweight and moderately obese persons is typically 70-80% fat and 20-30% lean tissues, however it is likely that the composition of weight loss in extremely obese persons may be different. We will analyze the composition of body weight in LABS patients undergoing rapid and large changes in weight using advanced body composition models and measurement methods. Using MRI, we will describe body composition changes at the tissue and organ level and adipose tissue distribution allowing us to address questions of biological and clinical importance including the body composition changes influences on REE. The Specific Aims are to: 1) identify the clinically important components of weight change in persons undergoing different types of bariatric surgery on fat mass and its distribution, and on fat-free mass (FFM); including bone mineral density (BMD), skeletal muscle, and specific organs at baseline and two time-points post surgery, and whether these are related to the type of surgery; 2) measure the change in REE following surgery induced weight loss and determine its relation to type of surgery, to changes in body composition compartments, and its duration over the follow-up. Secondary research questions relate to changes in cardiac structure and function, psychological functioning, and hormonal (ghrelin, leptin, insulin) levels associated with massive weight loss. Consenting, eligible LABS patients from Columbia University-Weill Cornell and the University of Pittsburgh will undergo some combination of the following measures: total body water by deuterium dilution (fat and FFM), extracellular water by sodium bromide tracer, body density by the BodPod, whole-body MRI (adipose tissue and its distribution; skeletal muscle mass; mass of liver, kidneys, heart, and brain), dual energy-X-ray absorptiometry (hip BMD, total body fat, FFM, and bone mineral content), and REE. Subjects will be African-American and Caucasian women and men (n=106; 50 percent (35>BMI kg/m2<45) will undergo MRI studies before surgery and one and two years later; 50 percent (45>BMI kg/m2<60) will have MRI only post surgery) equally distributed between the New York and Pittsburgh sites, and equally distributed across 3 surgical procedures: gastric banding; biliary pancreatic diversion and duodenal switch; and Roux-en-Y gastric bypass.